We've used Azamara & Holland America in the past. Azamara are high end and you pay for it - but brilliant nonetheless. Holland America are cheaper (but not cheap) and still an utterly excellent experience.
Avoid P&O at all costs.
Also, I don't think paying extra for a cabin with a window over an inside cabin is worth the money. It's balcony or nothing but you generally pay through the nose for it. Think about how much time you will actually spend in your cabin whilst not sleeping or in the shower.